Field
The present application relates generally to laser systems and, more specifically, to avoiding oscillation conditions in extreme ultraviolet light energy generated within a source plasma chamber.
Related Art
The semiconductor industry continues to develop lithographic technologies which are able to print ever-smaller integrated circuit dimensions. Extreme ultraviolet (“EUV”) light (also sometimes referred to as soft x-rays) is generally defined to be electromagnetic radiation having wavelengths of approximately between 10 and 100 nm. EUV lithography is generally considered to include EUV light at wavelengths in the range of 10-14 nm, and is used to produce extremely small features (e.g., sub-32 nm features) in substrates such as silicon wafers. These systems must be highly reliable and provide cost-effective throughput and reasonable process latitude.
Methods to generate EUV light include, but are not necessarily limited to, converting a material into a plasma state that has one or more elements (e.g., xenon, lithium, tin, indium, antimony, tellurium, aluminum, etc.) with one or more emission line(s) in the EUV range. In one such method, often termed laser-produced plasma (“LPP”), the required plasma can be generated by irradiating a target material, such as a droplet, stream or cluster of material having the desired line-emitting element, with a laser beam at an irradiation site within an LPP EUV source plasma chamber.
FIG. 1 illustrates some of the components of an LPP EUV system 100. A laser source 101, such as a CO2 laser, produces a laser beam 102 that passes through a beam delivery system 103 and through focusing optics 104 (comprising a lens and a steering mirror). Focusing optics 104 have a primary focus point 105 at an irradiation site within an LPP EUV source plasma chamber 110. A droplet generator 106 produces droplets 107 of an appropriate target material that, when hit by laser beam 102 at the primary focus point 105, generate a plasma which irradiates EUV light. An elliptical mirror (“collector”) 108 focuses the EUV light from the plasma at a focal spot 109 (also known as an intermediate focus position) for delivering the generated EUV light to, e.g., a lithography scanner system (not shown). Focal spot 109 will typically be within a scanner (not shown) containing wafers that are to be exposed to the EUV light. In some embodiments, there may be multiple laser sources 101, with beams that all converge on focusing optics 104. One type of LPP EUV light source may use a CO2 laser and a zinc selenide (ZnSe) lens with an anti-reflective coating and a clear aperture of about 6 to 8 inches.
For reference purposes, three perpendicular axes are used to represent the space within the plasma chamber 110, as illustrated in FIG. 1. The axis from the droplet generator 106 to the irradiation site 105 is defined as the x-axis (vertical in the example of FIG. 1); droplets 107 travel generally downward from the droplet generator 106 in the x-direction to irradiation site 105, although in some cases the trajectory of the droplets may not follow a straight line. The path of the laser beam 102 from focusing optics 104 to irradiation site 105 is defined as the z-axis (horizontal in the example of FIG. 1), and the laser beam 102 is moved or steered by the focusing optics 104 along the y-axis which is defined as the direction perpendicular to the x-axis and the z-axis.
In operation, the resulting EUV energy produced by the LPP EUV system 100 can experience oscillations which cause undesirable variations in wafer EUV light exposure. Further, a drifting of the focusing optics (caused by, for example, laser source power variation or focusing optics cooling water temperature variation) can cause the laser beam to slowly drift into a region of such oscillations. Rather than attempt to reduce or eliminate such oscillations, or directly address drifting focusing optics effects on laser beam positioning, what is needed is a way for the LPP EUV system 100 to continue operating by simply avoiding such issues.